U.S. and Iraqi forces killed two al Qaeda leaders this week in what U.S. officials described as a significant blow to the terrorist organization and promptly hailed as an Iraqi success story.

Abu Ayyub al-Masri, the military leader of al Qaeda in Iraq, and Abu Umar al-Baghdadi, head of the Islamic State of Iraq, were killed early Sunday in operations west of Tikrit, the hometown of the deposed dictator Saddam Hussein.

Al-Masri, an Egyptian also known as Abu Hamzah al-Muhajir, replaced the group’s Jordanian-born founder, Abu Musab al-Zarqawi, after al-Zarqawi was killed in June 2006. Al-Baghdadi, also known as Hamid Dawud Muhammad Khalil al-Zawi, served al Qaeda in Iraq as the leader of the Islamic State of Iraq and held the title “Leader of the Faithful.”

Al-Masri’s assistant and al-Baghdadi’s son, both of whom were involved in terrorist activities, also were killed in the operation.

Army Gen. Raymond Odierno, commander of U.S. forces in Iraq, said the deaths were “potentially the most significant blow to al Qaeda in Iraq since the beginning of the insurgency.”

“The government of Iraq, intelligence services and security forces supported by U.S. intelligence and special operations forces have over the last several months continued to degrade [al Qaeda in Iraq]. There is still work to do, but this is a significant step forward in ridding Iraq of terrorists,” he said.

A U.S. helicopter crashed during the operation, killing one U.S. soldier.

A U.S. intelligence official, speaking on the condition of anonymity because of the sensitive nature of the subject, said, “Getting these two guys off the street is significant for several reasons. They’re key figures in al Qaeda’s Iraqi affiliate. Al-Masri, an Egyptian, ran the show, with al-Baghdadi being its top local voice. Most of its people are Iraqi.”

The official said al Qaeda in Iraq has been taking “a pounding for years, and they’re more isolated than ever, in part because of their own brutality, but they can still launch deadly attacks.”

“Nobody thinks this is the end of their brand of extremism, but it’s a setback for them. And it was the Iraqis who led the way on this operation, an example of their skill in the face of a deadly, resourceful foe,” the official added. U.S. officials were quick to publicly credit Iraqi security forces with the mission’s success.

Vice President Joseph R. Biden Jr., describing the deaths of the terrorist leaders as “potentially devastating blows” to al Qaeda, said: “This action demonstrates the improved security, strength and capacity of Iraqi security forces. The Iraqis led this operation, and it was based on intelligence the Iraqi security forces themselves developed following their capture of a senior [al Qaeda] leader last month.

“In short, the Iraqis have taken the lead in securing Iraq and its citizens by taking out both of these individuals,” Mr. Biden said. “This counterterrorism operation is the culmination of a lot of cooperation and very hard work by Iraqi and U.S. forces to degrade [al Qaeda] over the past several months and years.”

Gen. David H. Petraeus, commander of U.S. Central Command, said the operation indicated the development of Iraqi capabilities.

Two terrorist leaders were “responsible for barbaric attacks that killed thousands of innocent Iraqi citizens and Iraqi and Coalition Security Force members,” Gen. Petraeus said. Congratulating U.S. and Iraqi troops, he said the terrorists’ deaths were “another major milestone in the effort to defeat extremism in Iraq” and “significant blows against extremism in Iraq.”

Iraqi Prime Minister Nouri al-Maliki announced the deaths at a news conference in Baghdad where he showed reporters photographs of the terrorists’ bloody corpses, the Associated Press reported. He said ground forces surrounded a house and used rockets to kill the two men, who were hiding inside.

Mr. al-Maliki described the deaths as “a quality blow, breaking the back of al Qaeda.”

The intelligence firm Stratfor noted in an analysis that al-Zarqawi had alienated many Iraqi Sunnis with his ruthlessness. “Al-Baghdadi is thought to have been largely a figurehead intended to reverse that alienation by putting an Iraqi face on al Qaeda in Iraq’s efforts, while al-Masri was considered the real brains and operational leadership behind [al Qaeda in Iraq]. It is his death that holds the most potential significance,” according to the analysis.

Stephanie Sanok, a senior fellow at the Center for Strategic and International Studies, said the developments were significant for two reasons.

“The first is that Iraqi intelligence and Iraqi security forces took the lead in this operation, with U.S. support,” she said. “The success is a shot in the arm for the Iraqis. It shows a real maturity that they didn’t have a little over a year ago.” Ms. Sanok served until last year at the U.S. Embassy in Baghdad, where she developed policy options for the U.S. government’s efforts to support a sovereign, stable and self-reliant Iraq.

The second point of significance was “the quickness and lack of equivocation of the U.S. response to this event,” Ms. Sanok said. Noting the remarks by Mr. Biden and Gen. Odierno, she said, “Rarely have we seen U.S. officials coming out so strongly in support of the Iraqis after such an operation.”

Under President Obama’s plan, all U.S. combat forces will leave Iraq by the end of August. The remaining force of about 50,000 U.S. troops will serve in roles as trainers and support personnel. In accordance with a U.S.-Iraqi security agreement, all U.S. forces will be removed from Iraq by the end of 2011.

In a separate operation, Iraqi forces arrested two suspected al Qaeda in Iraq associates northwest of Baghdad on Monday.

Ms. Sanok said the U.S. has been trying to push Iraqis into combat and combat-heavy operations. “It is going to be an increasing trend,” she added.